A friction clutch assembly or “clutch” of a car or other automobile having a manual transmission is generally located between the engine and the drive train. The assembly normally includes several adjacent annular plates, including a flywheel that is rotatably driven by the engine output (a crank shaft usually), a clutch plate (otherwise known as a driven plate), and a pressure plate that is biased by energy storing devices, such as one or more springs, towards the clutch plate and flywheel to clamp the clutch plate between the flywheel and the pressure plate.
The frictional engagement of the coupling faces of the clutch plate with the adjacent rotating coupling faces of the flywheel and the pressure plate allow the clutch plate to transfer power generated by the engine to the remainder of the drive train. However, unless there is some form of dampening in the drive line to dissipate the irregular impulses of the internal combustion petrol or diesel engine, these impulses will create unwanted driveline noise.
To prevent transmission of engine impulses through to the gearbox, clutch plates include drive springs for dampening purposes. The drive springs are typically coil springs. Thus, a typical clutch plate includes a splined hub that accepts a splined shaft to transmit engine rotation to the gearbox or transmission. The splined hub is connected to a hub flange, either rigidly or with some angular displacement for idle vibration dampening, and the hub flange is sandwiched between a main plate and a side plate which are fixed together. The hub flange is driven to rotate when the main plate is shifted into engagement with the flywheel of an engine through a friction material fixed to the circumferential edge of the main plate, The main and side plate assembly (hereinafter the “plate assembly”) and the hub flange are connected by drive springs to provide limited angular displacement between them. The angular displacement is provided to dampen torsional vibration in the drive mode of the vehicle (as compared to other vibration such as idle vibration).
The amount of dampening available is increased as the length of the drive springs increases. Therefore to increase the available dampening, the amount of angular displacement available between the hub flange and the plate assembly also must increase. Traditionally, drive springs are straight springs, and in the past, normally 3 or 4 springs are provided, spaced equidistantly about the splined hub. The preference for using straight coil compression springs arises on the basis that they are easy to manufacture and are therefore inexpensive. Straight drive springs have also been preferred to date because such drive springs can also operate without the need for guides along their length, to maintain them straight. This has the consequential benefits that the springs do not rub on other components of the clutch assembly, which would otherwise cause wear and generate heat, both of which can be detrimental to the life of the clutch plate.
The benefits of using straight coil compression springs as drive springs dissipate as attempts are made to provide a wider angle of displacement between the hub flange and the plate assembly. As indicated above, the length of the coil springs affects the amount of dampening which can be achieved. However, as the length of a straight drive spring is increased, the spring load can move out of alignment with the axis of the spring and that can reduce the normal compressive strength of the spring. The spring can actually be forced into a trapezoidal shape by the hub flange acting against the side and main plates of the clutch plate.
Applicant has previously recognised that benefits might be achieved by the adoption of curved drive springs in a clutch plate to increase the length of the drive springs. However, applicant also recognises that when curved drive springs are used to transmit engine torque, the springs are naturally forced radially outwards, bringing them into engagement with other parts of the clutch plate, including the clutch plate housing, causing them to rub against those parts. That rubbing can cause heat, wear, noise, erratic torque dampening and premature failure of the springs.
The present invention seeks to provide a clutch plate which has improved angular displacement between the hub flange and the plate assembly, compared to prior art clutch plates.